Endoluminal prostheses or endoprostheses are medical devices adapted to be implanted in a human or veterinary patient. Stents are a type of endoprosthesis which are deployed in blood vessel, urinary tract, bile duct, or other bodily lumen to provide structural support and optionally to deliver a drug or other therapeutic agent. Stents are generally cylindrical and function to hold open and sometimes expand a segment of the bodily lumen. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. Stents are often delivered to a desired location while in a reduced configuration having a smaller diameter than when fully deployed. The reduced configuration allows the stent to be navigated through very small passageways, such as coronary vessels and other bodily lumen. A crimping process is performed to place the stent in a reduced configuration. The stent can be crimped onto a catheter that can then be maneuvered over a guidewire that leads to a region of the anatomy at which it is desired to deploy the stent. The passageway through which the stent is maneuvered is often tortuous, so the stent should be capable of longitudinal flexibility. Once the stent has reached the desired deployment location, the stent is allowed to self-expand or is forcibly expanded by a balloon to an enlarged configuration. After deployment, the stent should maintain its enlarged configuration with minimal recoil back to its reduced configuration. All these functional requirements are taken into account in the structural design of a stent.
In addition to the foregoing functional requirements, it is also important for a stent to have the capability of being visualized to determine whether the stent has been properly maneuvered to the desired location and to confirm that the stent has properly deployed. Various imaging techniques, such as fluoroscopy and optical coherence tomography, may be used to obtain an image of the stent. Fluoroscopy uses X-rays while optical coherence tomography uses optical radiation.
Compared to metal stents, stents that have a polymeric substrate can be difficult to image due to their radiotranslucent and optically translucent properties. Structural features adjacent to the stent, such as parts of the anatomy and the catheter which carries the stent, can obscure the stent and make it difficult to ascertain its position. Radiopaque markers, such as metallic beads or metallic bands, can be embedded within or attached to the polymeric substrate so that the stent can be easily visualized using fluoroscopy. Radiopaque markers are relatively large in relation to the size of the stent substrate and can thereby affect stent function. Thus, stents often have only a few radiopaque markers which are strategically positioned.
Optical coherence tomography (OCT) has been used to obtain images that show individual stent struts. OCT typically employs near-infrared light which can penetrate through structures, such as biological tissue, which scatter the light. Interferometric analysis of the scattered light is used to generate images which can have a resolution in the order micrometers. International Application Publication No. WO 2010045386 A describes the use of OCT to obtain images in which reflective surfaces of metal stent struts can be identified. However, stent struts having a polymeric substrate are not as reflective as metal substrates.
OCT has been used to visualize stent struts made of a polymeric substrate. See Gutierrez-Chico et al., “Spatial Distribution and Temporal Evolution of Scattering Centers by Optical Coherence Tomography in the Poly(L-Lactide) Backbone of a Bioresorbable Vascular Scaffold” Circulation Journal, Vol. 76, 343-350 (February 2012). Gutierrez-Chico et al. describe the appearance of “scattering centers” or SC, which is defined as a “focal hyperintense backscattering signal” in the core of the stent strut. All the scattering centers were located exclusively at hinges. In a bench study, there was a complete absence of scattering centers in all regions of stents which were not subjected to crimping. After crimping and deployment, however, there were scattering centers in all hinge regions. Analysis of successive image slices through the hinges of an implanted stent showed that the scattering centers were located at the inner curvature of the hinge. Scattering centers were absent from image slices taken through the outer curvature of the hinge. As compared to the inner curvature of the hinge, parts of the stent which experienced little or no mechanical deformation during crimping and deployment appeared as “black boxes” within a dark field. The black boxes could be identified by a faint outline corresponding to the external surfaces of the stent structure.
There is a need for an imaging method, stent manufacturing method, and stent which allow for improved OCT imaging that can make it easier to determine where the stent structure begins or ends within a bodily lumen and make it easier to evaluate whether the stent has been properly deployed and is supporting surrounding tissue.